However, pie charts differ from the other common chart types in some significant ways and that makes the task of styling them a bit challenging. In particular, because styling happens at the DataPoint level (in the case of PieSeries, that's PieDataPoint: the visual representation of one of the slices of the pie), it initially seemed difficult to create a unified style for an entire pie because the sizes and positions of the individual slices can vary so dramatically for different data...

Using Absolute mode is obviously a big win, but there's still a significant limitation: the coordinates it uses are expressed in pixels and therefore are closely tied to the size of the pie. What looks good for a pie at one size looks silly for that same pie when it is made a little bigger or smaller. I called this limitation out with my initial demo and Pete mentions it in his post as well...

Knowing that Absolute mode works well to enable cohesive styling for fixed-size pies (just look at Pete's great second example!), it seemed to me that the sizing limitation could be overcome with some fairly simple code. So when I got a chance, I wrote that code and put together the following proof-of-concept demonstration using the existing examples from Pete's post (with only a few trivial tweaks to the XAML to adjust gradient offsets). What's great is that now both pies can both be resized dynamically and the styling looks good at any size!

[The complete sample code/project is available in PieDataPointMappingModeUpdater.zip as an attachment to this post.]

The principle here is simple: just update the relevant pixel coordinates of each gradient based on the current size of the PieSeries. What's more, a wrapper function abstracts out most of the complexity, so all that's necessary for the user is to pass in a simple helper method that's tailored to whatever specific pie design has been used. The helper is given the rectangular bounds of the pie and performs whatever gradient adjustments are necessary. It's that easy!

Here's what it looks like in code; the following is from the sample application's Page.xaml.cs:

Note that the constructor simply hooks things up and that each helper method is specific to the pie design it will be updating. All each updater does is nudge the pixel coordinates of its gradient to match up with the pie's size. So it seems reasonable to assume that other - potentially more complex - pie designs can be updated just as easily.

Here's the proof-of-concept wrapper method that does the bulk of the work:

And that's all there is to it! Now, not only do you know how to create appealing, holistic designs for fixed-size pie charts, you've got an easy way to keep those designs looking sharp for dynamically-sized pie charts as well. So go forth with that knowledge - and make even tastier pies! :)

Kind reader (and fellow Silverlight Charting blogger!) Pete Brown contacted me recently to report that my SilverlightDefaultStyleBrowser application (background reading available here, here, here, here, and here) didn't seem to be working for assemblies from the Silverlight Toolkit or Telerik's RadControls for Silverlight. Specifically, he found that SilverlightDefaultStyleBrowser would work successfully with the controls in the Silverlight runtime and in the Silverlight SDK - but when he used the "Add Assembly" button to add assemblies from the Toolkit or RadControls, no new controls appeared in the list. This was unexpected and I started investigating...

Silverlight Toolkit: This scenario was particularly troubling because I'm on the Toolkit team and I know it's not doing anything weird that should break SilverlightDefaultStyleBrowser. So I stepped through the code for parsing generic.xaml and discovered that the root ResourceDictionary wasn't getting loaded. A bit more debugging revealed that this was because the Silverlight Toolkit's generic.xaml uses a different XAML namespace than SilverlightDefaultStyleBrowser was looking for (and than nearly every other Silverlight assembly I've seen). But the Toolkit is not at fault here - it turns out that Silverlight supports two different XAML namespaces! So I added support for the second namespace to SilverlightDefaultStyleBrowser, and now it happily parses assemblies from the Silverlight Toolkit. :)

RadControls: I'd assumed the issue here was the same and expected the RadControls to "just work" now that I'd added support for the second XAML namespace - but I was wrong. :( So I stepped through SilverlightDefaultStyleBrowser's generic.xaml parsing code again (recall that generic.xaml is a public entry point for Silverlight assemblies) and discovered that SilverlightDefaultStyleBrowser was finding and parsing the RadControls generic.xaml just fine all along - except that there weren't any Style elements in it. So SilverlightDefaultStyleBrowser's behavior was "correct" in the first place! What's different here is that the Telerik assemblies expose a generic.xaml containing references to custom elements that work at run time (when generic.xaml is parsed and executed by Silverlight), but which do not work with the simple XML-level parsing that SilverlightDefaultStyleBrowser performs. I'm guessing it would be fairly straightforward to modify SilverlightDefaultStyleBrowser to successfully expose the Telerik Styles, but I'm not going to add that custom code until/unless I hear from someone at Telerik that this is something they're okay with. So (for now, at least), SilverlightDefaultStyleBrowser doesn't find any Styles in the RadControls assemblies; that behavior is correct and "by design".

Having investigated the problem report and fixed what I could, I updated the public SilverlightDefaultStyleBrowser application and the downloadable source code. And started writing this post! :)

The version number of SilverlightDefaultStyleBrowser appears in the window's title and the latest release number is 1.0.3268.34946. (Note: I haven't updated the screen shot below which shows the introductory version number.) If installed via ClickOnce, the application should automatically prompt you to upgrade when it detects the update (which typically happens after running the application once or twice). If you're using the standalone EXE, you'll need to update manually.

SilverlightDefaultStyleBrowser was written to do one thing and to do it simply. As is often the case when trying to duplicate existing behavior, there tend to be a few surprises along the way where things turn out not to work quite as expected. This was one of those surprises and I'm glad for the opportunity to fix this and make SilverlightDefaultStyleBrowser just a little more useful for everyone!

Added support for new series type BubbleSeries. To keep thing simple and avoid needing to specify another set of values, the bubble size is bound to the dependent value of the series.

Added support for the new Independent(Range|Category)Axis and Dependent(Range|Category)Axis properties of the Column/Bar/Line/Scatter/Bubble series types. Off by default, either axis can be enabled by checking the relevant box in the settings panel for a series - then customized as always.

Added a ToolTip for the "Axes" header label to display the current value of the Chart.ActualAxes property in pseudo-XAML. More of a diagnostic aid than anything, this information can help understand the interaction between axes and series - and what's really going on behind the scenes.

Increased the maximum number of stand-alone Axis instances from 2 to 4 now that multiple axes are supported.

Various other improvements.

Updated the version to 2008-12-07.

ChartBuilder continues to be tremendously useful to me as an interactive Charting test application. The plethora of knobs, dials, and switches that represent a user experience designer's worst nightmare [ :) ] enable a degree of scenario testing that has helped find and fix a wide variety of issues. Furthermore, the interactive nature of the tool - and the ease with which it enables mocking up samples for answers on the Silverlight Controls and Silverlight Toolkit forum - has really seemed to help people understand Charting more easily.

My hope is that ChartBuilder can be just as useful to you - so if you've got ideas after using it, please let me know!

The December 08 release of the Silverlight Toolkit was published a short while ago. Just about every control in the Toolkit got some love and attention for this release and I encourage you to have a look, download it, and enjoy!

That said, readers of my blog know that I'm all about the Charting. :) So just like my original Charting announcement and overview post, here's an announcement of the December 08 Charting release and an overview of some of the high points. By now, I assume folks have had time to play around with Charting and are starting to get into some more advanced scenarios, so the samples are going to be a little more technical than before. If you want a refresher on basic Charting concepts, please have a look at my original overview - all of those concepts still apply.

Though the Charting team was down to just two of us for this release, Jafar and I have done our best to deliver some pretty compelling new features. It's important to note that there are a few breaking changes from the November 08 release, but I hope you'll agree that the new functionality is worth the minor inconvenience of updating existing Charting code. As it happens, if you don't explicitly use the Axis class, you probably won't need to change anything at all!

I wrote the following summary for the release notes, and it seems like a good way to begin:

Notable Changes

Support has been added for arbitrary numbers of Axes for a Chart. A key and significant consequence of this change is that it is now possible to mix previously incompatible Series in the same chart (example: Column and Bar). The former "automatically share axes when possible" behavior remains present and can be used to render two series with a shared independent axis and different dependent axes in order to display related-but-differently-scaled values (example: an engine's torque and RPM).

The Column/Bar/Line/Scatter Series classes expose two new (optional) properties which specify a particular Axis instance to be used by the Series (rather than letting the Series choose from the Chart's Axes collection as it does by default). These properties allow very specific customization of each Series' axes in situations where maximum control is desired.

A design-time assembly for Charting has been created which is automatically used by Blend and Visual Studio to enhance the design-time experience. Most class properties are now categorized into custom "Data Visualization" and "Data Visualization Styles" categories to make them easier to find - while particularly common properties are now found in the "Common" category. ToolTips identify the purpose of each control and property, and Toolbox icons are automatically associated with each class. (Note: Some of these features are only supported by one of the design tools.)

The design-time behavior of DataPoints has been enhanced: DataPoints are now visible by default when dropped onto the design surface in Blend, so styling them is easier. PieDataPoint now creates a default Geometry (that can be customized interactively with the ActualRatio and ActualOffsetRatio properties) which makes it possible to see and understand relevant styling changes. DataPoints in a Chart show up in Visual Studio without a refresh of the design surface.

The Chart class is now decorated with ContentPropertyAttribute("Series") which means that it is no longer necessary to explicitly wrap Series with <charting:Chart.Series> ... </charting:Chart.Series> in XAML. Additionally, this enables Blend to display a Chart's Series in the "Objects and Timeline" pane and makes accessing the properties of a Series considerably easier. Example of simplified XAML syntax:

<charting:Chart>
<charting:PieSeries ... />
</charting:Chart>

The behavior of the Axis classes during animations that expand or shrink the range of displayed data has been changed so that the animation of the size change is smooth (vs. jumping between axis intervals). This significantly increases the ease with which dynamic data changes can be observed and understood by the viewer.

The DataPointSeries class (a subclass of Series) has been unsealed to make the task of writing a custom Series considerably easier. While this change doesn't expose the entire hierarchy on which the "in-box" Series are built, it is a significant benefit to developers because DataPointSeries implements many of the key Series notions: the ItemsSource property, DataPoint creation, dynamic data detection, change animation, show/hide transitions, DataPoint selection, and more.

A new Series type, BubbleSeries, has been added (along with its associated BubbleDataPoint class). BubbleSeries is similar in nature to ScatterSeries, but conveys additional information by using the size of the data points to display an additional dependent value for each of the data points.

In scenarios where multiple columns/bars (of ColumnSeries/BarSeries) share the same category, all such data points will now be automatically displayed overlapping so that smaller values will no longer be obscured from view by larger values.

Breaking Changes

The Axis.AxisType property has been replaced by a corresponding hierarchy of Axis classes. The concrete classes map directly to the AxisType values that were removed: LinearAxis, DateTimeAxis, CategoryAxis. In addition to improving overall API consistency, a consequence of this refactoring is that the LinearAxis.Minimum/Maximum properties are now strongly typed as double and the corresponding properties on DateTimeAxis are strongly typed as DateTime - both of which improve development-time and design-time usability.

The Axis.ShouldIncludeZero property has been removed; this property is has no meaning for some axis types and is typically not needed because the same thing can be accomplished by setting Minimum or Maximum accordingly. A consequence of this is that Column/Bar charts will usually not include the 0 value by default (and therefore the "beginnings" of the columns/bars can be truncated). This behavior is consistent with Excel (demonstrated by creating a column chart of the values 10, 11, and 12 in Excel), though truncation is more likely because Charting's heuristics are more aggressive with regard to excluding 0.

Other Changes

Various UI improvements and bug fixes.

Whew - there's a lot of good stuff in there! :)

So now that we have an idea what's new, let's start by looking at the support for multiple axes with a common scenario: charting two quantities that are related, but measured in different units and/or on different scales. For the purposes of this demonstration, let's chart the performance characteristics of an imaginary engine:

[Note: Complete source code for all of the sample charts here can be found in the ChartingIntroduction.zip file attached to this post.]

We start with the usual Chart object to contain the series - but now we can put the series directly inside the Chart tags instead of inside nested Chart.Series tags (though that syntax still works and is fully supported). There are two LineSeries here (one for torque and one for power) and each starts out as you'd expect by hooking up to the data and doing a bit of customization. But then there's something new: the LineSeries.DependentRangeAxis property is used to identify a specific LinearAxis. Every series type (other than PieSeries which doesn't have axes) now exposes a Dependent???Axis property and an Independent???Axis property (where "???" is "Range" or "Category" depending on which series it is).

Normally, a series will search the Chart.Axis collection to find an axis that it can use - but when these new properties are set, it always uses the specified axis. So in this example we're providing a specific LinearAxis for each LineSeries to use and we're customizing it slightly to get the chart looking just how we want. Having specified both LineSeries in this manner, a third LinearAxis is added to the Chart.Axis collection where it will be found - and used - by both LineSeries when they acquire an independent value axis.

Moving on, here's a chart using the new BubbleSeries in a financial scenario (for a fictional stock ticker symbol). It's the usual "stock price by date" chart we've all seen before - but this chart is also showing the trading volume each day by varying the size the data points:

BubbleSeries is used just like a ScatterSeries, but it exposes an additional property SizeValueBinding that works just like Independent/DependentValueBinding to identify the source of the size values. The DataPointStyle property specifies a custom style for BubbleDataPoint (not shown here) where we've added the day's volume to the ToolTip of each bubble. This custom style was created in the usual manner - by starting from the default style for BubbleDataPoint and making the desired changes. (I did so in Visual Studio's XAML editor, but it could just as easily be done in Blend.)

Tweaking things just a little more, the independent value axis is customized by setting its AxisLabelStyle property and providing a style for the new DateTimeAxisLabel class. DateTime/NumericAxis/AxisLabel are the classes used to display an axis's labels. In this case we've provided a custom StringFormat so that the dates are all displayed in the friendly Month+Day pattern seen above. DateTimeAxisLabel offers specific StringFormat properties for each supported IntervalType - but in this case we've simply used the StringFormat property which conveniently overrides the other, more specific types.

Next up is a fairly typical column chart showing made-up bowling scores for some of the people on the Toolkit team. But there's twist because there are two "Shawn"s on the team:

This is a standard ColumnSeries, which is specifying a IndependentCategoryAxis so the new SortOrder property of CategoryAxis can be used to automatically sort the category names. This data set contains two items with the independent value "Shawn". Previously, both would have been displayed in the same category slot with the same width which means the column in front could have obscured the one in back and led viewers to believe that only four scores were being displayed. But now ColumnSeries and BarSeries automatically detect this situation and overlap columns/bars in the same category slot (after sorting them by size). This behavior makes it clear that five values are being shown and makes it easy to hover over either of the "Shawn"s to get a ToolTip with the associated score.

The previous chart is interesting for another reason, too: it's using the natural, simple way to display the data, but a purist might argue that it's also the "wrong" way to do so... [Jafar and I actually had this "argument" - I won't say who took which position. :) ] Categorizing by player name works well enough, but what if we also had an image for each player and we wanted to display that image instead of their name? Changing the ColumnSeries.IndependentValueBinding property might work, but it's become obvious that we're mixing display concerns and data concerns.

When maintaining developer/designer separation is critical, what's probably more appropriate is to categorize by the actual data object - and then display it however we want. Here's a chart that does just that:

And here's the XAML - which is a bit more complicated than before, but still quite manageable:

Now that we've set IndependentValueBinding to the data objects themselves, there are automatically five categories (one for each of the five objects in the data set). (Aside: CategoryAxis will still sort them for us because the data objects implement the IComparable interface.) Once the data is set up "properly", all that's left to do is customize how the objects are displayed - and this is done by providing a custom template for the AxisLabel class (something that's already been discussed).

One other thing to note above is how the Chart.Title property is set to a tree of UI elements instead of the usual string. Chart.Title (and other such properties in Charting) follows the ContentControl model because it makes simple things simple while also allowing more advanced scenarios. In this case, all we've done is provide a "subtitle" - but we could also have gone much further and created something very customized.

The last thing I wanted to highlight was some of the design-time enhancements for Charting in this release. Data points were previously a little tricky to customize because their default state is not visible (which allows the reveal/show animation to fade them in without flicker). Data points now check if they're being used in design-time and will automatically play their reveal/show animation so that they're visible on the design surface by default. PieDataPoint used to be extra tricky because its Geometry property is only set when it's actually being used by PieSeries. But now it knows about design-time and its ActualRatio/ActualOffsetRatio properties can be changed to interactively evaluate different styling approaches. Here's a stand-alone PieDataPoint in Blend showing off both of these improvements (along with the new "Data Visualization" property category):

PS - If you're looking for an update to my ChartBuilder application (Background reading: Introduction, Update) to show off the new Charting functionality, please stay tuned because it will be available very soon! :)

PPS - I'm on vacation in December, so responses to blog comments and emails may be delayed. But please don't let that stop you from contacting me: I'll follow up on everything I get - I just might be a little slower than usual. :)